1. Field of the Invention
The present invention relates to an image composition method or the like for producing an image signal having a wide dynamic range by composing together a high-sensitivity image signal and a low-sensitivity signal, which have been obtained by means of photographing a subject, and more particularly, to an image composition method, a solid-state imaging device, and a digital camera, which hardly cause brightness inversion and are suitable for producing a composite image having a high signal-to-noise ratio.
2. Description of the Related Art
An image photographed by a solid-state imaging device using a solid-state imaging element typified by a CCD image sensor or a CMOS image sensor has a shortcoming of a narrow dynamic range. For this reason, a photographic scene involving a wide dynamic range; for instance, optical images such as an image of a backlighted person or an afternoon garden observed by way of window panes from a dark, indoor location, cannot be acquired well.
Therefore, as described in JP-A-6-141229 (corresponding to U.S. Pat. No. 5,455,621), the related-art digital camera is equipped with a solid-state imaging element capable of controlling a period of accumulation of electric charges. An image signal obtained as a result of a subject having been exposed for a long period of time (hereinafter called a “long-exposure image signal”) is read, and an image signal obtained by exposing the same subject for a short period of time (hereinafter called a “short-exposure image signal”) is continuously read. These two image signals are subjected to composing operation, to thus produce an image of the subject having a wide dynamic range.
According to the related-art technique described in JP-A-59-210775, there is used a solid-state imaging element, wherein half of an array of pixels is formed as high-sensitivity pixels and a remaining half is formed as low-sensitivity pixels. An image signal obtained from the high-sensitivity pixels and an image signal obtained from the low-sensitivity pixels, both image signals having been obtained during the same exposure time, are merged together, thereby producing an image of the subject having a wide dynamic range.
A long-exposure image signal output from the solid-state imaging element or an image signal obtained from the high-sensitivity pixel is called a high-sensitivity image signal. Moreover, a short-exposure image signal or an image signal obtained from the low-sensitivity pixel is called a low-sensitivity image signal.
Digital data constraints are usually imposed on an image signal to be processed by a digital camera. For instance, even when a high-sensitivity image signal and a low-sensitivity image signal, not yet having been merged, are 8-bit signals, realization of an image signal of a composite image as a signal of a total of 16 bits is impossible in terms of costs. Therefore, composing operation must be performed such that an image signal of composite image having a wide dynamic range becomes an 8-bit signal.
Therefore, a composite image having a wide dynamic range suffers a drawback of a partial reduction in gradation difference of the image and the image becoming less-defined and monotonous.
For these reasons, according to the related-art technique described in JP-A-2002-135649, composite image data are generated at respective pixel positions on the entire screen through use of a gamma characteristic shown in FIG. 9. Namely, a high-sensitivity image signal is subjected to nonlinear conversion processing through use of a gamma characteristic I (solid line) which drastically suppresses a high-level region. A low-sensitivity image signal is subjected to nonlinear conversion processing through use of a gamma characteristic II (broken lines). The high-sensitivity image signal and the low-sensitivity image signal, which have undergone conversion processing, are added together, thereby producing a composite image signal. The thus-produced composite image signal expresses a gradation difference in a range in which losing color is induced as a result of saturation of the high-sensitivity image signal and a gradation difference in a range in which the low-sensitivity image signal becomes solid, thereby producing a well-defined composite image signal.